Synergetic role of strain relaxation and photoexcitation for enhanced nanoscale piezoelectricity in freestanding BaTiO 3 thin films

Strain engineering has emerged as a strategic approach for modulating the piezoelectric and ferroelectric properties of thin films. We investigated local piezoelectric properties in clamped and freestanding epitaxial BaTi O 3 films via piezoresponse force microscopy. The freestanding membrane exhibits a ∼ 300 % increase in the effective piezoelectric coefficient compared to the clamped films, driven by increased nonlinearity. Quantitative analysis reveals about fourfold increase in the nonlinearity parameter, originating from enhanced domain wall motion enabled by strain relaxation after removal of substrate-induced clamping. Our molecular dynamics simulations further support about threefold increase in domain-wall velocity in the freestanding film. Additionally, under optical excitation, the membrane exhibits a photoinduced enhancement of the piezoelectric response arising from coupled photostriction effect and light-driven domain wall motion. Density functional theory calculations reveal ∼ 2 % decrease in the out-of-plane lattice parameter of the freestanding membrane under optical excitation, consistent with the observed photoinduced deformation. However, the corresponding change in the clamped film ceases to exist. These results identify substrate clamping as a critical limitation to piezoelectric performance and establish freestanding ferroelectric membranes as promising platforms for low-power electromechanical and optoelectronic devices.